Technology used for connecting steel pipes used in equipment of the oil production industry such as oil country tubular goods, riser pipes, and line pipes includes threaded joints. Threaded joints for steel pipes are constituted by a pin having a male thread element provided at the end of a first tubular member and a box which is a female thread element provided at the end of a second tubular member. The joint is tightened by engaging the male thread and the female thread.
Standard threaded joints are prescribed by API (American Petroleum Institute) standards but in recent years, the environments for exploration and production of crude oil and natural gas are becoming more severe. As a result, there is increasing use of high performance special joints referred to as premium joints which are not prescribed by API standards.
In a typical premium joint, in addition to a tapered thread for firmly connecting steel pipes, the pin and the box have sealing surfaces which can form a metal-to-metal seal for providing sealing performance and torque shoulder surfaces for acting as stoppers during tightening (make-up).
In the past, since vertical wells were predominant, it was sufficient for a threaded joint for OCTG to resist the tensile load due to the weight of pipes and to prevent leakage of high pressure fluid passing through it. However, in recent years, wells are becoming deeper, and deviated wells and horizontal wells which bend underground are increasing, and the development of wells in difficult environments such as in the sea or in polar regions is increasing. Therefore, the properties required for threaded joints are becoming more varied and more strict as exemplified by resistance to compression, resistance to bending, ability to seal against external pressure, and ease of use in the field.
As a result, qualification tests for threaded joints are becoming more severe. In the Series A test of recent ISO 13679 standards; alternances of internal pressure and external pressure combined with tension or compression (internal pressure+tension, internal pressure+compression, external pressure+tension, and external pressure+compression) (herein referred to as repeated combined loads) are applied three times. Such severe test conditions were not taken into consideration in the past in the development of threaded joints.
As shown in FIG. 2, a typical premium joint has a structure in which an unthreaded portion 12 referred to as a lip portion is provided at the end of a threaded portion having a tapered male thread 11 provided on a pin 1 which is a male threaded element. A sealing surface 13 for forming a metal-to-metal seal is provided on the outer peripheral surface of the lip portion. A torque shoulder surface 14 is provided on the end surface of the lip portion (and accordingly on the end surface of the pin).
Naturally, a box 2 which is a corresponding female threaded element is provided with a female thread 21, a sealing surface 23, and a shoulder surface 24 at the rear of the box which correspond to or mate with the male thread 11, the sealing surface 13, and the end surface shoulder surface 14, respectively, of the pin 1.
An overlap in the radial direction referred to as interference is provided between the sealing surfaces of the pin and the box. This type of threaded joint is designed so that if the joint is tightened until the shoulder surfaces of the pin and the box contact each other, the sealing surfaces of both members are in intimate contact around their entire periphery and sealing is provided by metal-to-metal contact.
The above-described sealing performance is exhibited to the highest degree when tightening is carried out with a suitable torque in the period from when the shoulders abut until the shoulders begin to undergo plastic deformation (when a normal tightened state occurs).
The shoulder surfaces act not only as stoppers for tightening but also act to bear almost all of a compressive load acting on the joint. Therefore, if the shoulder surfaces are not thick or if the shoulders are not stiff, they cannot withstand a large compressive load.
Prior art for increasing the resistance to external pressure and resistance to compression of a premium joint is described in WO 2004/109173 (Patent Document 1). That threaded joint greatly increases resistance to external pressure by providing a portion which does not contact the box (referred to below as a nose portion) between the shoulder surface and the sealing surface of the end surface of the pin. At the same time, the taper angle of the nose portion is made 0 degrees (a cylindrical surface) or is made smaller than that of the sealing surface. Due to the provision of the nose portion, a decrease in the thickness of the end surface shoulder surface is prevented, and an increase in resistance to compression can be achieved.
However, in the threaded joint described in Patent Document 1, when a high compressive force and external pressure are simultaneously applied, and when a tensile force and internal pressure are subsequently simultaneously applied, it has been found by the inventors that there can be a risk of developing a leak.
WO 00/08367 (Patent Document 2) discloses a threaded joint in which a tight contacting region is provided in two locations of a lip portion, namely, a tight contacting region near the threaded portion is defined as a sealing surface and a tight contacting region near the end surface (shoulder surface) is defined as a protecting portion. The protecting portion which is a second contact region provided in a location close to the end of the lip portion has the object of supplying a first seal to internal pressure (and thus to protect the sealing surface) and to optimize the forces and moments undergone by the lip.
In the threaded joint described in Patent Document 2, the amount of interference of the protecting portion is set to be higher than the amount of interference of the sealing surface (at least 1.15 times and at most 1.3 times) in order to retain a sufficient amount of contact pressure on the protective portion while a moderate contact pressure is obtained on the sealing surface. In addition, as Patent Document 2 incites to set sufficient distances from the shoulder to the protective portion, from the protective portion to the sealing surface and from the sealing surface to the thread, the lip portion is designed to be extremely long, so a sufficient thickness (radial dimension) of the shoulder surface may not be obtained. As a result, when a high compressive load is applied, the joint may not sufficiently withstand the compressive load, and it is thought that its compressive performance may be inadequate.
DE 4446806 (Patent Document 3) discloses a threaded joint of the same type as disclosed in Patent Document 2.
U.S. Pat. No. 4,473,245 (Patent Document 4) discloses a threaded joint in which a metal-to-metal seal is provided on the exterior of the pipe and a torque shoulder provides an additional metal-to-metal seal. In the threaded joint disclosed in Patent Document 4, however, the thickness of a lip portion is designed to be very thin, so it is difficult to ensure the resistance to high external pressure and high compression.
U.S. Pat. No. 3,489,437 (Patent Document 5) discloses a threaded joint in which a metal-to-metal seal and a shoulder are provided. However, in the threaded joint disclosed in Patent Document 5, the lip portion is designed according to the same rule as a typical premium joint in FIG. 2, so it is not considered to ensure the resistance to high external pressure and high compression.
U.S. Pat. No. 3,870,351 (Patent Document 6) discloses a threaded joint characterized by its shoulder surfaces having a particular profile. In the threaded joint disclosed in Patent Document 6, the shoulder surfaces are rounded profiles (the shoulder surface of one of the pin and box being convex and that of the other being concave), and they function to form a second seal. This structure is intended to equalize the contact between the sealing surfaces of the first seal by suppressing misalignment or the flexure of the lip toward the pipe axis at the time of make-up.
However, in a threaded joint disclosed in Patent Document 6, although equalization of the sealing contact or sealing pressure of the first seal is taken into account, it is out of consideration to use the threaded joint in a situation where a high compression force and high external pressure simultaneously act thereon. As shown in FIG. 7 of this patent document, there is a space between the outer side of the concave shoulder surface of the box and the outer edge of the convex shoulder surface of the pin. In a situation where a high tension force and high external pressure act thereon, the shoulder surfaces of pin and box can separate easily due to the high tensional force and the tip of lip can easily be deformed or be moved towards the outer side (that is, in the direction which expands the diameter). Therefore, in a threaded joint disclosed in Patent Document 6, it is difficult to control and suppress the deformation or movement of the tip of the pin towards the outer side as described above. Furthermore, if the shoulder surface of the pin has a convex profile, the innermost edge part of the shoulder surface of the box which is concave is so thin that the shoulder surface of the box tends to undergo a heavy plastic deformation when a high compression load is applied to the joint.
Patent Document 1: WO 2004/109173
Patent Document 2: WO 00/08367
Patent Document 3: DE 4446806
Patent Document 4: U.S. Pat. No. 4,473,245
Patent Document 5: U.S. Pat. No. 3,489,437
Patent Document 6: U.S. Pat. No. 3,870,351